This invention relates to a method of making an article having at least one re-entrant surface.
Rapid Prototyping of plastics by various techniques is well known and more recently metal powders have been processed by laser in layers. It is equally well known that support structures (‘scaffolding’) are frequently required and these may be provide in the same material or by a secondary material that may be e.g. water soluble or melts at a lower temperature to the primary material used to build the desired prototype article. It is also known to form an article by using adhesive droplets deposited on each thin powder layer to create bonding. Such a process is “3D printing” as described in U.S. Pat. No. 6,589,471. Also known is where a ‘green’ article is first formed e.g. in Metal Injection Moulding whereby metal powder is bonded together by a binder which is then fired to form the nominally solid article. These are inherently never 100% dense and are not useful for many applications—particularly where internal voids can precipitate fatigue cracking. Such bonding/firing techniques may be used to form metal parts from a metal/polymer powder raw material where firstly the polymer is melted/sintered by selective application of a laser to form a ‘green’ article that is subsequently fired to metal and join together the metal powder. Alternatively the article formed of powder may not be fully dense and it is subsequently infiltrated with a liquid that hardens to make a solid object.
More particularly it is known to manufacture articles by laying down successive layers of metal powder, and, after each layer has been laid down, solidifying selected portions of the layer by suitable operation of a scanning laser so that an article of simple or complex shape can be built up. Some form of support or ‘scaffold’ is frequently required to support a layer of the article during build and there is a significant problem with any structure in which the solidified portion is not supported by a solidified portion in the preceding layer, because if the laser is applied to a bulk of unsolidified metal powder, that powder simply balls up into clumps analogous to ‘weld splatter’. Such solid supports of certain complex structures, particularly where they are located one above another, present great difficulty. Whilst they can be built, their subsequent removal to complete the article may be difficult or impossible. An example is where two rows of vanes are to be built one above another within a double walled casing.